Cobalt-substituted myoglobin and hemoglobins have been examined by equilibrium and kinetic techniques in order to define the ligand binding characteristics and to compare with natural iron-containing counterparts. Due to EPR visibility of the cobalt system, the chemical nature, stereochemical structure, and their correlation of the metal-ligand bonding have been determined in detail. It is determined that the ligand in hemoproteins may change its geometry upon freezing, so that careful monitoring of such a movement is needed in defining the electronic and stereochemical structure.